AN EXPERIMENTAL STUDY OF FE-AL SOLUBILITY IN THE SYSTEM CORUNDUM-HEMATITE UP TO 40 KBAR AND 1300°C

Abstract

The mutual solubility in the system corundum–hematite [[α-(Al, Fe3+)2O3] was investigated experimentally using both synthetic and natural materials. Mixtures of γ-Al2O3 and α-Fe2O3 (weight ratios of 8:2 and 10:1) were used as starting materials for synthesis experiments in air at 800–1300°C with run times of 7–34 days. Experiments at 8–40 kbar and 490–1100°C were performed in a piston-cylinder apparatus (run times of 0.8–7.4 days) using a natural diasporite consisting of 60–70 vol.% diaspore and 20–30 vol.% Ti-hematite. During the diasporite–corundite transformation, the FeTiO3 component (12–18 mol%) of Ti-hematite only slightly increased, implying that oxygen fugacity was maintained at high values. Run products were studied by electron microprobe and X-ray diffraction (Rietveld) techniques. An essentially linear volume of mixing exists in the solid solution with a slight positive deviation at the hematite side. Up to 1000°C, corundum contains <4 mol % Fe2O3 and hematite <10 mol% Al2O3; at 1200°C these amounts increase to 9.3 and 17.0 mol %, respectively. At 1300°C hematite was no longer stable and <f>?-<fen><cp type="lpar">Al0·88Fe0·123+O3<cp type="rpar"></fen></f> coexists with the orthorhombic phase <f><rm>Fe</rm>0·533+<rm>Al</rm>0·47<rm>O</rm>3</f>. The present results agree with corundum (solvus) compositions obtained in previous studies but indicate a larger solubility of Al in hematite. The miscibility gap in the solution can be modelled with an asymmetric Margules equation with interaction parameters (2? uncertainties): <f><rm>W</rm><rm>H</rm>Cor = 89 ± 10?kJ/mol</f>; <f><rm>W</rm><rm>S</rm>Cor = 23 ± 8?J/<fen><cp type="lpar">Kmol<cp type="rpar"></fen></f>; <f><rm>W</rm><rm>H</rm>Hem = 91 ± 14?kJ/mol</f>; <f><rm>W</rm><rm>S</rm>Hem = 14 ± 10?J/<fen><cp type="lpar">Kmol<cp type="rpar"></fen></f>. Application of the corundum–hematite solution as a solvus geothermometer is limited because of the scarcity of suitable rock compositions.